A coupling pin is a cylindrical surface connector commonly made of metal. Many coupling pins have parallel crossway holes through either end where keys can be inserted to hold them in place. Custom fitted coupling pins may only be held in place by surface pressure. Coupling pin keys are usually wire posts or nails bent into a ring shape at one end. The coupled joint is generally designed be disassembled quickly by pulling the keys from the coupling pin.

The coupling pin is commonly installed with half its length making contact with each surface. This position evenly distributes the pressure of holding the connection and makes the attachment to each surface equal. Using the wrong coupling pins may cause them to fit poorly and bend or break unexpectedly. Coupling pins that are not secured at either end with keys may slip out of position over time.

Coupling pins are sometimes used as fail-safes on connected rotating parts. A fail-safe feature is designed to break under certain conditions. The planned failure of coupling pins is a common method of preventing more costly damage to the other parts of a system. Fail-safe coupling pins typically resist breaking during normal operation of the coupled joint. Improper installation of a coupling pin may decrease its potential to protect the other moving parts from damage.

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Joints with coupling pins can usually be pulled apart easily for cleaning and servicing. The work load of some coupling pins can warp them with regular use. Coupling pins are often inspected and may be replaced during scheduled mechanical check-ups.

Specialized coupling pin varieties are used to strengthen connected joints. Pin lug couplings are hollow cylinder connectors that come in both internally and externally threaded variations. Internal threading is often used to hold the two halves of a coupling pin together. External threading on coupling pins is frequently twisted into the two surfaces to bring them together. Rubber grommets are regularly used to ensure a snug fit on coupling pins.

Scaffolding is almost always connected with strong steel coupling pins. The quickly assembled metal structures used by construction workers is called scaffolding. Long steel poles have pre-drilled holes where a coupling pin can be inserted to make a temporary connection. Coupling pins are locked into position with spring-loaded tension clips at both ends. Pulling the coupling pins out in the right sequence allows for the scaffolding structure to be disassembled safely.

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